a) Field of the invention
The present invention relates to an illumination system for endoscopes such as fiber scopes, non-flexible endoscopes and video endoscope.
b) Description of the prior art
Since endoscopes are inserted into body cavities, etc. for observing the interiors thereof, the locations to be observed through the endoscopes are normally in utter dark. It is therefore necessary to equip the endoscopes with illumination systems of their own.
An optical system of an ordinary endoscope is illustrated in FIG. 18 wherein a fiber scope is selected as an typical example. As shown in this drawing, the optical system of the endoscope is roughly divided into an observation optical system 11 and an illumination optical system 12. The observation optical system consists of an objective lens 13 which is disposed in the distal end of the endoscope for forming an image of an object, an image guide fiber bundle 14 for transmitting the image and an eyepiece optical system 15 for allowing observation of the image transmitted through the image guide fiber bundle 14. On the other hand, the illumination optical system 12 consists of a light source device 18 comprising a light source lamp 16 and a condenser lens 17, a light guide fiber bundle 19 for transmitting the light from the light source device 18 to the distal end of the endoscope and an illumination lens 10 for irradiating the object with the light emerging from the light guide fiber bundle 19.
In the recent years where endoscopes are needed in various fields of application for observing and inspecting interiors of body cavities, machines, etc., various types of endoscopes having different field angles, outside diameters, lengths, etc. are manufactured. Under this circumstance, a variety of illumination systems equipped with light guides made of optical fibers having different numerical apertures (NA's) are utilized.
Since the light guides having large NA's allow light bundles having large NA's to be incident thereon for transmission therethrough, these light guides can transmit large amounts of lights per unit sectional area thereof and have large distribution angles. However, since the optical fibers composing the light guides use, in the cores thereof, glass materials which have relatively large refractive indices and high degrees of pigmentation, they have a common tendency to tint the transmitted light yellowish, and this tendency is more remarkable as the light guides become longer.
On the other hand, light guides having small NA's does not allow lights having large NA's to be incident thereon for transmission therethrough, thereby transmitting small amounts of light per unit sectional area thereof and having small distribution angles. However, the light guides having small NA's permit using, in the cores of optical fibers, glass materials having relatively small refractive indices and transmit lights in conditions rather white as compared with those transmitted through the light guides having large NA's.
As a light source device using a light guide, there is known the light source device for endoscopes disclosed by Japanese Patent Kokai Publication No. Hei 1-144009. In this light source device, the light which is emitted from the light source optical system thereof and incident on the end surface of incidence of the light guide has an NA fixed at a unique level.
When the light emitted from a light source device has an NA fixed at a unique level, as in the above-mentioned conventional example, which optimized for a light guide having a large NA, it is possible to perform illumination effectively with the light guide by making use of the merits thereof, i.e., the wide distribution angle and the large amount of transmission light per unit sectional area thereof. In contrast, a light guide having a small NA will not transmit a light bundle having a large NA emitted from a light source device and cause loss of light amount.
Further, when a light bundle emitted from a light source has an NA fixed at a unique level which is optimized for a light guide having a small NA, it is possible, by using a light guide having a small NA, to obtain an illumination light closer to the white light without causing loss of light amount. If a light guide having a large NA is used with such a light bundle, however, the merit of the light guide to transmit a large amount of light per unit sectional area cannot be made use of, or the light will be transmitted in the amount substantially the same as that obtainable with a light guide having a small NA, and the illumination obtained will be tinted yellowish.
As is understood from the foregoing description, aptitudes, NA's and diameters of light bundles which are emitted from light source devices and incident on light guides are different dependently on NA's and sectional areas of light guides built in endoscopes.